The present invention relates to a noise detector circuit for detecting noise contained in a received signal.
Noise detector circuits are used as a control circuit for squelching a receiver or for selecting a receiver of highest S/N ratio in a diversity receiving system. They also find application in the electric field intensity measurement.
A conventional noise detector circuit is generally composed of a high-pass filter for deriving noise from a received signal, a rectifier (containing a diode) for rectifying the output of the filter, an integrator (consisting of a resistor R and a capacitor C) for integrating the output of the rectifier, and a comparator either for comparing the output of the integrator with a given reference detection level or for converting the output signal into a digital signal and then comparing the digital signal with a given reference digital signal, to provide a detection signal.
However, such a noise detector circuit has the following disadvantages:
(a) Since the noise detector circuit is generally composed of analog circuits, it is sensitive to variations of various circuit constants. Particularly, the forward voltage characteristic of the diode in the rectifier is affected directly by the change in temperature, deteriorating accuracy of the noise detection.
(b) A high-pass filter for deriving noise is required.
(c) The integrator having an inherent time constant slows down the noise detection.
(d) Since the characteristics of the high-pass filters and rectifiers employed in the noise detector circuits are different from one detector circuit to another, delicate adjustment is needed in respective noise detector circuits to make the overall noise detection level uniform. In addition, the reference detection level is apt to be changed by the variation in the power source voltage.
(e) Where a digital comparator is used, an A/D converter must be used to convert the analog output signal of the integrator into a digital signal.
For further details of the prior art techniques relating to the above-described noise detector circuits, reference is made to the following published literatures:
(1) U.S. Pat. No. 3,783,384 (issued Jan. 1, 1974); PA1 (2) U.S. Pat. No. 3,613,004 (issued Oct. 12, 1971); PA1 (3) T. Nagatsu, et al "Base Station Radio Equipment for 800 MHz Band Land Mobile Telephone System", Review of the Electrical Communication Laboratories, Vol. 25 No. 11-12, pp. 1245 to 1254, 1977; PA1 (4) William G. Figel, et al "Vehicle Location by a Signal Attenuation Method", IEEE Transactions on Vehicular Technology, Vol. VT-18, No. 3, pp. 105-109, November 1969; and PA1 (5) T. Nagatsu "Vehicle Locating by Field Detection in the Small-Cell Land Mobile System", Review of the Electrical Communication Laboratories, Vol. 21, No. 11-12, pp. 830 to 842, 1973.